The US State Department has issued a worldwide travel alert for its citizens, warning of the potential for "anti-US action" after the US accused Iran of backing a plot to kill Saudi Arabia's ambassador to Washington.
Iran has angrily denied the US accusations, saying Washington was mounting an offensive to try to sow discord between Tehran and its Arab neighbours.
Officials in Tehran called the allegations a "prefabricated scenario" and "stupid mischief" meant to divert attention from US woes at home and in the Middle East.
US President Barack Obama called the alleged plot a "flagrant violation of US and international law" and Saudi Arabia said it was "despicable."
At a news conference, FBI Director Robert Mueller said the convoluted plot - involving monitored international calls, Mexican drug money and an attempt to blow up the ambassador in a Washington restaurant - could have been straight from a Hollywood movie.
Attorney-General Eric Holder claimed that the plot was the work of the Islamic Revolutionary Guard Corps and the Quds Force, its covert, operational arm.
The US State Department said the alleged plot "may indicate a more aggressive focus by the Iranian government on terrorist activity against diplomats from certain countries, to include possible attacks on the United States".
"US citizens residing and travelling abroad should review the Department's Worldwide Caution and other travel information when making decisions concerning their travel plans and activities while abroad," it said.
The alleged plotters have been identified as Manssor Arbabsiar and Gholam Shakuri, both originally from Iran, in the criminal complaint unsealed in federal court in New York City.
Mr Arbabsiar, a naturalised US citizen, was arrested in late September. Mr Shakuri is still at large.